Are you sure you want to maintain two mortgages while renovations are done? I realize it can be a PITA to live through construction, but it might not be as disruptive as you imagine. For example, basement renovations -- meh, would that even affect you? Same goes for moving walls around or opening other parts of the house -- no big deal. If you work, you'll likely be gone during the day when most of the construction is happening anyhow.
Bathrooms and kitchens are more problematic, but not that horrible to live through. I moved into our current house and then we had major renovations done. It wasn't so horrible. If you have a decent contractor that minimizes the unavailability of major systems and moves in an orderly fashion (for example, keep one bathroom intact while gutting and renovating the other), it's really not so bad. If you are really opposed to being there while the kitchen is torn apart for a week, you could likely move into an extended-stay hotel for that time at probably 1/10th the cost of 3+ months of dual mortgages. For us, it was no big deal to use a microwave for a few days and wash dishes in another sink.
The significant upside of course is not having to pay both mortgages (particularly if renovations go slower than planned, which is almost always). Also, being on site to monitor daily work and progress is another advantage. And, as you're noting, you won't have to worry at all about bridging the two house loans.
